Bearing materials

ABSTRACT

A bearing and a bearing alloy composition are described, the bearing alloy comprising in weight%: tin 5-10; copper 0.7-1.3; nickel 0.7-1.3; silicon 1.5-3.5; vanadium 0.1-0.3; manganese 0.1-0.3; balance aluminium apart from unavoidable impurities.

[0001] The present invention relates to bearing materials comprisingaluminium alloys bonded to a strong backing material.

[0002] Highly rated internal combustion engines have conventionally usedcrankshaft bearings comprising either a copper based alloy or analuminium based bearing alloy bonded in some manner to a strong backingor substrate material such as steel for example. The actual workingsurface of the bearing alloy, i.e. that surface which faces the enginecrankshaft journal surface has also been provided with a so-calledoverlay coating which is a thin coating of a relatively softer metalalloy such as lead-tin, lead-tin-copper or lead-indium for example. Thepurpose of the overlay coating is to provide conformability and dirtembeddability properties to the bearing. Conformability is that propertyof a bearing which allows it to accommodate slight mechanicalmisalignments between the bearing and shaft surfaces and is a measure ofthe ability of the overlay alloy to distribute the applied load. Dirtembeddability is that property which allows debris particles in thelubricating oil to be embedded in the soft overlay alloy without causingdamage such as scoring of the shaft. Whilst the technical advantages ofoverlay coated bearings are not disputed they have the significantdisadvantage of being expensive to make due to the overlay generallybeing deposited by electroplating which is a relatively very labourintensive process.

[0003] Manufacturers of motor vehicles are more frequently asking forbearings which do not have overlay coatings as they are cheaper to buy.However, some engines whilst not possessing a particularly high specificoutput, due to their design, impose high loads on the crankshaftbearings or possess particularly thin oil films between the bearing andshaft journal and are consequently prone to “scuffing” of the bearingsurface. Scuffing is where metal to metal contact between the crankshaftjournal surface and the bearing surface occurs, i.e. the oil film at thepoint of contact is ruptured allowing metal to metal contact. Scuffingrelates to momentary metal to metal contact without actual seizure andconsequent failure of the bearing. However, whilst overlay coatedbearings are especially scuff resistant, most of the conventional copperand aluminium based alloys are relatively poor in terms of scuffresistance. The ability to withstand scuffing is a measure of theconformability of the alloy. In contrast to scuffing, seizure is relatedto lack of compatibility of the alloy.

[0004] One known material comprising aluminium-6 wt% tin-1 wt% copper-1wt% nickel has good scuff resistance but has a relatively low fatiguestrength and toughness in the non-overlay plated condition which rendersit unsuitable for more modern highly rated engines. The low fatiguestrength and toughness is a reflection of the low ductility of thisalloy.

[0005] To cope with the stresses imposed by modern engines, an alloyhaving significantly improved mechanical properties, viz. tensilestrength (15%); hardness (15%); and fatigue strength (16%) than one ofthe strongest known aluminium bearing alloys comprising: aluminium-12wt% tin-4 wt% silicon- 1 wt% copper which is in a solution heat treatedform, is required. Whilst the strength of this alloy could be raised byincreasing the copper content it is difficult and expensive to make bythe usual production methods of casting billets and rolling to size androll-pressure bonding to steel owing to the small size reductions whichare possible at each rolling pass before annealing heat treatment isrequired.

[0006] GB-A-2271779 describes an aluminium/tin/silicon bearing alloywhich may further comprise at least one of the elements Mn, Mg, V, Ni,Cr, Zr, and/or B at between 0.1 and 3.0 weight% per element. In additionto these elements, the alloy further contains 0.2 to 5.0 weight% Cu, 0.1to 3.0 weight% Pb, 0.1 to 3.0 weight% Sb and 0.01 to 1.0 weight% Ti asadditional alloying elements. It is explained that if the content of theoptional elements Mn, Mg, V, Ni, Cr, Zr and B rises above 3.0 weight%the conformability of of the bearing may deteriorate and workability ofthe bearing alloy can be degraded.

[0007] GB-A-2266564 is also concerned with aluminium-based bearingalloys similar to GB'779 described above. In this case the alloy alsopreferably includes At least one or two further elements of from 0.2 to5.0 weight% Cu, from 0.1 to 3.0 weight% Pb, from 0.1 to 3.0 weight% Sb,Mn, Mg, V and Ni and 0.01 to 1.0 weight% Ti, the total amount of Mn, Mg,V and Ni being in the range from 0.01 to 3.0 weight%.

[0008] However, alloys made according to the teachings of the above twodocuments are virtually unprocessable by the normal production methodsof casting and rolling followed by roll-pressure bonding due to lack ofductility and brittleness of the alloys. This is the case when thealloying element contents are a small fraction of those quoted.

[0009] It is an object of the present invention to provide an aluminiumalloy having greater strength and scuff resistance than known alloyswhilst retaining ease of manufacture.

[0010] According to a first aspect of the present invention, there isprovided a bearing alloy composition comprising in weight%: tin 5-10;copper 0.7-1.3; nickel 0.7-1.3; silicon 1.5-3.5; vanadium 0.1-0.3;manganese 0.1-0.3; balance aluminium apart from unavoidable impurities.

[0011] Preferably, the tin content lies in the range from 5.5-7 weight%.

[0012] Bearing testing has surprisingly shown that when the siliconcontent falls below 1.5wt% then the incidences of seizure increases.When the silicon content exceeds 3.5% then the silicon network tends tobe coarser and the incidence of cracking during alloy processing, byrolling for example, increases significantly necessitating additional inprocess heat treatments and smaller rolling reductions per pass thus,increasing the cost of production. Preferably, the silicon content ismaintained within the range from 2 to 3 wt%.

[0013] The additions of copper and nickel are well known strengtheningadditions for aluminium alloy bearing materials. Additions below 0.7 wt%do not produce the required strengthening effect whereas additions above1.3 weight% render the alloy difficult to process. At higher contents ofcopper and nickel, only relatively small rolling reductions are possiblebefore annealing heat treatments are required which increases the costof the material.

[0014] Vanadium has the effect of increasing the toughness of the alloy.Below 0.1 weight% the effect diminishes rapidly whereas above 0.3weight% there is an embrittling effect. Preferably, the vanadium contentis maintained at a maximum of 0.2 weight%.

[0015] Manganese in addition to being a chemical alloy strengthener, isa well known grain refining agent producing smaller grains and hencegreater strength than would be the case without it. Below 0.1 weight%the grain refining effect is small whereas above 0.3 weight% manganese,alloy processing becomes difficult and expensive necessitating reducedrolling reductions per pass and additional heat treatments.

[0016] We have found that the combination of the two additional elementsof vanadium and manganese in small quantities within the limitsprescribed above has a synergistic effect wherein the strength of thealloy is raised significantly and, as importantly, the conformabilityand compatibility of the alloy are not adversely affected to anysignificant extent. Tests have shown that the alloy shows improvedfatigue strength and resistance to scuffing at comparable loads to knownstrong aluminium alloys whilst retaining ease of manufacture and lowprocessing costs.

[0017] However, in the types of engine applications for which this alloyis intended it is the combination of the increased mechanical strengthproperties together with improved scuff resistance and acceptableseizure resistance which is the surprising effect of the alloycomposition of the present invention.

[0018] According to a second aspect of the present invention, there isprovided a plain bearing comprising a strong backing material and havingbonded thereto a layer of a bearing alloy having a compositioncomprising in weight%: 5 tin 5-10; copper 0.7-1.3; nickel 0.7-1.3;silicon 1.5-3.5; vanadium 0.1-0.3; manganese 0.1-0.3; balance aluminiumapart from unavoidable impurities.

[0019] The bearing may also include an interlayer of relatively purealuminium or an aluminium alloy material between the bearing alloy andthe strong backing material.

[0020] The strong backing material may be steel or bronze for example.

[0021] It has been found that the ductility of the alloy containing bothvanadium and manganese is significantly greater than alloys containingonly one of these additions. It is believed that this feature isresponsible for the improved scuff resistance of the material.

[0022] Whilst the material of the present invention is primarilyintended to be used in relatively highly loaded engines prone toscuffing due to low oil film thickness under arduous operatingconditions for example, it will be appreciated by those people skilledin the bearings art that this material would operate perfectlysatisfactorily with an overlay coating of the type describedhereinabove.

[0023] In order that the present invention may be more fully understood,an example will now be described by way of illustration only withreference to the accompanying drawings, of which:

[0024]FIG. 1 shows a cross section through part of a bearing utilisingthe alloy of the present invention and showing the constituent layers;

[0025]FIG. 2 is a histogram showing relative scuff resistance resultsfor an alloy according to the present invention and for threecomparative alloys;

[0026]FIG. 3 shows a similar histogram to that of FIG. 2 but showingrelative seizure results for the same alloys; and

[0027]FIGS. 4A to 4C which show a part cross section of test apparatusfor determining scuff and seizure ratings and graphs indicating the testregimes for fatigue (4B) and scuff/seizure (4C) testing.

[0028]FIG. 1 shows a cross section through part of the circumferentiallength of a substantially semi-cylindrical half bearing through a planenormal to the axis of the bearing. The bearing 10 comprises a steelbacking layer 12 having a layer 14 of the bearing alloy thereon with athin interlayer 16 of relatively pure aluminium therebetween. Theproduction process for the bearing will be understood from the exampleproduction schedule described below.

[0029] Bearings produced from the material described above were formedinto half bearings for testing. The bearings had a wall thickness 1.75mm comprising a steel thickness of 1.5 mm and a lining thickness of 0.25mm.

[0030] Other comparative alloys having a composition as set out in Table1 below were made into bearings of the same dimensions and tested underthe same conditions. TABLE 1 Composition Material Sn Si Ni Cu V Mn AlComparative 1 12 4 — 1 — — Bal Comparative 2 20 — — 1 — — BalComparative 3 12 4 — 2 — — Bal Inventive alloy  6   2.5 1 1 0.2 0.25 Bal

[0031] Mechanical properties of the above alloys are set out below inTable 2. TABLE 2 Mechanical Properties Lining % Elong Al Grain HardnessUTS to Toughness Size Material (HV2.5) (Mpa) Fracture * (μm) Comparative1 47 150 20 20 17 Comparative 2 40 120 23 18 16 Comparative 3 47 150 1819 17 Inventive Alloy 52 180 21 25 12

[0032] It may be seen that the inventive alloy is not only stronger thanthe comparative alloys but has lost no ductility relative to alloys 1and 3 which are also aluminium-tin-silicon-copper alloys.

[0033] The bearings were tested to determine the fatigue strengththereof, the load at which scuffing occurred and the ultimate load atwhich seizure occurred. The tests were carried out in a known Sapphireapparatus as shown in FIG. 4A. The apparatus 20 comprises a test shaft22 having a central eccentric portion 24 supported by the test bearings26, 28, the outer ends of the shaft are supported in slave bearings 30,32. The shaft is rotated by a drive motor 36 and load is applied to thetest bearings 26, 28 by a connecting rod 40 to which is applied a forceby a piston 42 which is actuated by hydraulic means 46, 48. Straingauges 50 measure the applied load. FIGS. 4B and 4C show typical regimesfor fatigue and scuff/seizure testing. The fatigue load capacity is thatload which causes fatigue at 200 hours running. In operation, theapparatus shown at FIG. 4A applies a load to the test bearings 26, 28 bymeans of the eccentric portion 24 and the hydraulically loaded piston 42thus imposing a sinusoidal dynamic load on the bearings. Via a computercontrol system (not shown), a programmed progressive load increasebecomes the basis of the measurement of surface properties. In this modeof increasing load, the minimum oil film thickness steadily reduces andthe test measures, via the temperature increase, the load at which thematerial is wiped or scuffed as it comes into contact with thegeometrical inaccuracies in the shaft and/or the load at which thematerial welds itself to the shaft. Scuff resistance is a measure ofmaterial conformability whilst seizure resistance is a measure ofcompatibility.

[0034]FIG. 4B shows an illustrative schematic graph showing that as theload on a bearing increases, the number of cycles which it can withstandprior to fatigue diminishes. FIG. 4C illustrates a scuff/seizure testingschedule. An increasing load is applied to a test bearing until scuffingor seizure occurs. Scuffing or seizure is generally indicated by a risein temperature at the bearing surface. Scuffing tends to be a momentarytemperature rise whereas seizure is a prolonged temperature riseaccompanied by a fall in oil pressure.

[0035] The test results are shown below in Table 3. TABLE 3 RelativeRelative Sapphire L-N Sapphire Sapphire Fatigue 200 hr scuff seizureMaterial Load Capacity resistance resistance Comparative 1 1.14 0.740.95 Comparative 2 1 1 1 Comparative 3 1.14 0.53 1 Inventive Alloy 1.340.81 0.81

[0036] The relative bearing properties shown in Table 3 are based on 18tests for the inventive alloy and a minimum of 60 tests for each of thecomparative alloys. In the Table Al20Sn1Cu alloy (comparative alloy 2)is given a base-line rating of 1 against which all the other alloys,including the inventive alloy, are rated. Thus, the fatigue strength ofthe inventive alloy is 34% greater than comparative alloy 2, forexample.

[0037] As may be seen from Table 3, the fatigue strength of the alloyaccording to the present invention is significantly higher than thethree comparative alloys and although lower in actual seizure resistanceit also has improved scuff resistance relative to the other knownAl/Sn/Si comparative alloys 1 and 3. The results shown in Table 3 arealso depicted graphically in FIGS. 2 and 3.

[0038] In essence the material according to the present invention has asignificantly greater fatigue strength than known alloys whilstretaining an entirely adequate resistance to both scuffing and seizure.Thus, the alloys according to the present invention are particularlyuseful for those engines requiring a higher fatigue strength and scuffresistance than known silicon containing alloys but which do not requirean especially high seizure resistance rating.

1. A bearing alloy composition comprising in weight%: tin 5-10; copper 0.7-1.3; nickel 0.7-1.3; silicon 1.5-3.5; vanadium 0.1-0.3; manganese 0.1-0.3; balance aluminium apart from unavoidable impurities.
 2. A bearing alloy according to claim 1 wherein the tin content lies in the range from 5.5-7 weight%.
 3. A bearing alloy according to either claim 1 or claim 2 wherein the silicon content is maintained within the range from 2 to 3 wt%.
 4. A bearing alloy according to any one preceding claim wherein the vanadium content is a maximum of 0.2 wt%.
 5. A bearing alloy substantially as hereinbefore described with reference to the accompanying description and drawings.
 6. A plain bearing comprising a strong backing material and having bonded thereto a layer of a bearing alloy having a composition comprising in weight%: tin 5-10; copper 0.7-1.3; nickel 0.7-1.3; silicon 1.5-3.5; vanadium 0.1-0.3; manganese 0.1-0.3; balance aluminium apart from unavoidable impurities.
 7. A plain bearing according to claim 6 , the bearing further including an interlayer of relatively pure aluminium or an aluminium alloy material between the bearing alloy and the strong backing material.
 8. A plain bearing according to either claim 6 or claim 7 wherein the strong backing material is selected from steel and bronze.
 9. A plain bearing according to any one of preceding claims 6 to 8 wherein an outer surface of the bearing alloy is provided with and overlay coating layer.
 10. A bearing substantially as hereinbefore described with reference to the accompanying description and drawings. 